Question

A long, straight wire of radius 5.0 mm carries a current of 20A. i) Calculate the
magnetic field at the surface of the wire, and ii) calculate the perpendicular distance,
from the axis of the wire, at which the magnitude of magnetic field will be half of
its value at the wire surface.

Answer 1

i) The magnetic field at the surface of the wire:
B=(μ_0 I)/2πR=(4π∙〖10〗^(-7) (20))/2π(0.005) =0.0008 T=0.8 mT.
ii)
B~1/r
Thus,
r^'=2r=10 mm.